1. Field of the Invention
This invention relates to the production of coated porous mineral fillers, and more particularly, to a method of pressure impregnating hydrophobic coatings onto porous mineral fillers.
Minerals are utilized as fillers and extenders in compounding polymeric compositions. For example, porous mineral fillers such as calcium carbonate, calcium sulfate, expanded perlite or vermiculite, talc, mica and the like are being used in polymeric composites in increasing portions to obtain superior properties as well as to decrease cost of production by using less polymeric material in the filler-polymer matrix.
Some of these filler materials have limited utility however due to their porous nature, their surface being a fine network of cracks and fissures, and/or due to their water solubility. Most are prepared by comminuting an appropriate ore or rock to a particular grade of particle sizes as by crushing and/or grinding and then classifying with high speed air separators. This causes considerable cracking and fissuring of the particle's surface. Some filler materials are roasted or calcined before, during, or after the comminuting step which introduces further stressing of the particle's surface. Others, notably vermiculite and perlite are subsequently heated to their softening temperature range, expanded or exfoliated by internal gas pressure, and are then reclassified. The reclassification step frequently crushes a portion of the exfoliated structure and allows channeling access to otherwise enclosed voids within the structure. Thus, for example, an expanded perlite may contain many cracks and fissures communicating with otherwise sealed interior air filled cavities. When used in polymeric latex compositions, such as a paint or ready-mixed joint compound, it is desirable that the filler maintain water resistance for long periods of time e.g. up to two years. Any pinhole perforations or fissures into the interior air-filled cavities of the perlite particles will lead to gradual water penetration, causing the perlite to lose its desirable characteristic as a lightweight filler well before the paint or joint compound is to be used, and which may lead to early separation of the latex emulsion.
Other filler materials have limited utility due to gradual water solubility. Fillers having significant water solubilities of about 0.04% or greater, such as calcium carbonate, wollastonite, calcium sulfate anhydrite and gypsum dihydrate, frequently allow capillary channels to form in the polymeric composite after exposure to moisture. For example, calcium sulfate, as the deadburned anhydrite form, only very gradually gives up the last vestige of its water of crystallization and appears to regain it readily upon a few weeks or months storage. This makes some deadburned anhydrites unsuitable for compounding in uses where any residual moisture causes problems. Further, the general water solubility of calcium sulfate anhydrite at room temperature restricts its use in other polymer substrate compounding applications.
2. Description of the Prior Art
It is known in the filler art to apply certain coatings via solution or dispersion onto mineral fillers by soaking the filler in a vat of coating liquid followed by draining and drying the coated filler; and also by pouring or spraying the coating liquid onto a tumbling bed of filler followed by drying and curing. For example, U.S. Pat. No. 3,769,065 discloses a method for coating expanded perlite wherein the material is sprayed first with an acid solution, next with a water glass solution followed by another acid solution, and finally dried and heat cured. U.S. Pat. No. 4,255,489 discloses coating and then heat curing expanded perlite with polysiloxane. U.S. Pat. No. 4,141,751 discloses spraying a silane or titanate coupling agent onto extremely fine inorganic particles undergoing frictional, shearing pressure agitation to generate curing heat in a high intensity mixer. U.S. Pat. No. 2,714,578 discloses obtaining controlled reduced permeability in the interior pores of coarse porous material such as sintered ceramic material, metal powder, or sandstone by first evacuation then elevated temperature and pressure injection of molten paraffin wax or low melting point metal alloys in the production of controlled diffusion barriers for use in a diffusion cell.